There are three types of electrolytic cells primarily used for the commercial production of halogen gas and aqueous alkali metal hydroxide solutions from alkali metal halide brines, a process referred to by industry as a chlor-alkali process. Two of these cells are the diaphragm cell and the membrane cell. The general operation of each cell is known to those skilled in the art and is discussed in Volume 1 of the Third Edition of the Kirk-Othmer Encyclopedia of Chemical Technology at page 799 et. seq., the relevant teachings of which are incorporated herein by reference.
In the diaphragm cell, an alkali metal halide brine solution is continually fed into a anolyte compartment containing an anolyte solution where halide ions are oxidized at the anode to produce halogen gas. The anolyte solution, including alkali metal cations contained therein, migrates to a catholyte compartment containing a catholyte solution through a hydraulically-permeable microporous diaphragm disposed between the anolyte compartment and the catholyte compartment. Hydrogen gas and an aqueous alkali metal hydroxide solution are produced at the cathode. Due to the hydraulically permeable nature of the diaphragm, the anolyte solution mixes with the alkali metal hydroxide solution formed in the catholyte compartment.
The membrane cell functions similarly to the diaphragm cell, except that the diaphragm is replaced by a hydraulically-impermeable, cationically-permselective membrane which selectively permits passage of alkali metal ions to the catholyte compartment. The membrane essentially prevents hydraulic permeation of the anolyte solution to the catholyte compartment, except for the alkali metal cations. Therefore, a membrane cell produces alkali metal hydroxide solutions relatively uncontaminated with the alkali metal halide brine.
Membrane cells are typically assembled in "stacks" comprising a plurality of bipolar plate electrodes, the electrodes being assembled in a filter press arrangement wherein each electrode is positioned in a spaced-apart but face-to-face planar relationship with respect to an adjacent electrode. A membrane is positioned between each adjacent bipolar electrode, thereby forming a series of alternating catholyte and anolyte compartments. A stack may also comprise a plurality of membrane cells having monopolar electrodes where the cells are electrically connected in series with respect to each other. Membrane cell stacks generally have common electrolyte and product piping. Membrane cell stacks are known in the chlor-alkali industry and, for example, are described in Volume 6A of Ullman's Encyclopedia of Industrial Chemistry (5th Ed. 1986) at pages 399 et. seq., the relevant teachings of which are incorporated herein by reference.
It is well known that shunt currents exist in stacks of bipolar plate cells with common electrolytes. These shunt currents are undesirable for at least two reasons: they can cause corrosion of some of the components of the system, and they are currents that are essentially lost in terms of the production of the desired products of the system. The corrosion problem can be particularly severe if the shunt currents leave the cells via conducting nozzles to which they are attached the inlet and outlet tubes for the cells. It is desirable, therefore, to be able to reduce the effect of the shunt currents for all of the inlet and outlet tubes for cells in stacks.
The piping carrying the anolyte or brine to the stack of the cells is normally a titanium containing metal which is connected to the stack by a non-conductive tubing. During normal electrolysis, shunt currents pass from the individual cells at the positive end of the stack and enter the tubes. When the tubes are made of a poor electrical conductor, the current flow that passes in the tubes is conducted by ions. This current is also called the bypass current. The current operates by a cathodic electrolysis reaction such as: EQU 2H.sub.2 O+2e.sup.- .fwdarw.H.sub.2 +2OH.sup.- E.degree.=-0.2V (1)
At the negative end, the current leaves the piping by an anodic electrolysis reaction, such as: EQU 2Cl.sup.- .fwdarw.Cl.sub.2 +2e.sup.- E.degree.=+1.3V (2)
The current then flows from the housing at the positive end into the non-conductive tubes and returns to the cells at the negative of the cell stacks. The current flow in the non-conductive tubes is again conducted by ions and in order for the current to enter the metal structure at the negative end of the cell stack, a reduction reaction such as reaction (1) must again occur.
Because of these shunt currents, titanium may be dissolved by an anodic reaction such as: EQU Ti+4Cl.sup.- .fwdarw.TiCl.sub.4 +4e.sup.- E.degree.=+0.4 (3)
Merely grounding the titanium piping as proposed by the prior art does not solve the problem of protecting the titanium against corrosion as a result of the shunt currents since they still exist and corrosion can still occur at those points where the current flows. TiH+ forms as a result of penetration of atomic hydrogen into titanium and typically occurs as a result of an electrolysis reaction. TiH+ is known to cause embrittlement of titanium.
An important member of an electrolyzer system to be protected is the titanium nozzle which is connected to the anolyte compartment at one end and is connected to the polymeric or Teflon tubing leading to the titanium piping at the other end. Shunt currents pass through this nozzle which is located at the negative end of the cell stack. To prevent a reduction reaction that produces hydrogen and creates TiH.sub.2, corrosion protection should be provided. Since the nozzle is a piping member that must contain Cl.sub.2 and anolyte under pressure, its protection against TiH.sub.2 stress crack failure is important.